The present invention generally relates to methods and devices for improving the stability, flexibility, and/or proper anatomical motion of a spinal column and more particularly, to spinal implant devices for use between adjacent vertebral bones.
The normal human spine contains 23 moveable intervertebral discs located between the adjacent vertebral body endplates of the spine. These discs form an important part of the articulating systems of the spine, allowing for complex motion. In general, the discs permit movements such as flexion, extension, lateral flexion, and rotation. Discs are living tissue but have no blood supply. Disc tissues are sustained by an exchange of waste products and nutrients with surrounding vascular tissues. This exchange is augmented by increases and decreases in pressures within the disc tissues.
Intervertebral discs undergo anatomical changes including degeneration due to natural aging processes and due to injury. Disc degeneration is a progressive process and can include a decrease in the water and proteoglycan content of the nucleus pulposus and the annulus, distortion of the collagen fibers of the annulus fibrosus, and tears in the lamellae.
Disc degeneration is problematic, because degeneration of the intervertebral discs reduces the ability of the discs to perform their various functions such as absorbing and distributing load forces of the spine vertebrae. Essentially, the degenerated discs no longer function as effectively as shock absorbers. Additionally, disc degeneration can result in narrowing of the invertebral spaces, resulting in additional stresses to other spinal components, particularly the ligaments of the spine. Narrowing of the intervertebral disc spaces can also result in spinal segment instabilities. In more serious cases of disc degeneration, a disc can become wholly degenerated resulting in adjacent spinal vertebrae coming in contact with one another, a painful condition associated with numerous adverse and serious complications of the spine. All of these changes can lead to abnormal motion of spinal segments and pain during normal physiological movements.
Intervertebral disc degeneration is treated with many modalities, including methods that focus on disc replacement, regrowth or stimulation of the degenerated discs, and spinal immobilization and stabilization devices. Surgery is often employed in extreme cases when instability or pain develops or when there is a compromise of neural elements of the spine. Historically, surgery has been designed to remove degenerative discs, modify the anatomy of the spine to accommodate the degenerative processes, replace disc components with synthetic material, or fuse adjoining vertebrae to prevent painful movement.
One example of a disc replacement device is the “Fernstrom ball,” which is essentially a ball placed in between vertebrae to maintain an appropriate height between the vertebrae. Such disc replacement devices suffer from a variety of disadvantages including subsidence of the device into vertebral end plates. In other words, over time, the ball can poke through and into the adjacent vertebrae thus losing any increase in height of the disc space. The “Fernstrom ball” is also rigid and acts as a barrier to the disc tissues from the needed changes in intradiscal pressure needed to sustain living cells. The “Fernstrom ball” is usually made of steel, which can have adverse reactions with tissues and whose trace metals may interfere with cell proliferation and rejuvenation.
Regeneration of discs may be facilitated by the transplant of more normal disc material from adjacent healthier discs, autologous grafts, and/or by the introduction of growth factors or other stimulants to aid in disc regeneration. For example, one author proposes the use of adult mesenchymal stem cells to stimulate regrowth of intervertebral discs. See, e.g., Steck et al., Induction of Intervertebral Disc-Like Cells from Adult Mesenchymal Stem Cells, 23 STEM CELLS 403-411 (2005). Because of the early stage of some of these methods however, this solution is not ideal for all degenerative disc problems. Furthermore, in the degenerative disc, compensatory anatomical changes may have taken place. Loss of elasticity and compressibility coupled with Modic changes in adjacent bony endplates may predispose the discs to further deterioration. Adjacent ligaments may have contracted and thickened, further isolating disc tissues from necessary nutrients.
Another approach uses immobilization devices to isolate and stabilize the vertebrae affected by the degenerated intervertebral disc. Some of these devices use bolts and screws to immobilize adjacent vertebrae of the spine. This solution suffers from a number of disadvantages including reduced mobility of the spine. Additionally, immobilizing two adjacent vertebrae has the disadvantage of transferring stresses to adjoining levels of vertebrae thus accelerating the degenerative process in adjacent vertebrae. Physiologic motion is lost with frequent decreases in activity level to include vocational and recreational practices.
Some conventional approaches have proposed replacing the intervertebral disc with a synthetic device. The synthetic implants suffer from a number of disadvantages. In conventional synthetic implant procedures, all remaining normal disc material is removed and excluded from the disc space. Failed attempts to replace herniated or degenerated nuclei include the concepts of a waterbladder (See U.S. Pat. No. 3,875,595), a hydrophilic elastomer (See Edeland, Suggestions for a total elasto-dynamic intervertebral disc prosthesis, 9 BIOMATER. MED. DEV. ARTIF. ORGANS 65-72 (1981)), and a silicone polyethylene implant (See Edeland, Some Additional Suggestions for an intervertebral disc prosthesis, 8 J. BIOMED. MATER. RESOURCES APPL. BIOMATER. S36-S37 (1989)). Other reported problems of synthetic implants such as the one disclosed in Ray, The PDN® Prosthetic Disc-Nucleus Device, 11 (Suppl. 2) EUR. SPINE J. S137-142 (2002), include difficulties in implantation techniques as well as reports of implant dislocations. Some of the prior art synthetic disc replacement devices heretofore proposed have taught that the replacement device should be rigid and/or not compressible. By not being compressible, the rigid prior art disc replacement devices fail to adequately perform the natural functions of intervertebral discs, including acting as a shock absorber to absorb and distribute the forces imposed by the vertebrae.
Although a variety of solutions have been proposed to address the problem of invertebral disc degeneration, the prior art solutions to the problem of degenerative discs heretofore proposed suffer from one or more disadvantages, including among others, failing to provide an environment conducive to regeneration of normal intervertebral disc material, failing to restore normal intervertebral disc function, and/or failing to sustain normal physiological function of the person and biological function of the disc itself.